Update git-merge-flow-config.jsonc (dotnet#11139)

Author: JanKrivanek

.config/git-merge-flow-config.jsonc

@@ -30,9 +30,13 @@
         "vs17.11": {
             "MergeToBranch": "vs17.12"
         },
-        // MSBuild latest release to main
+        // Automate opening PRs to merge msbuild's vs17.12 (SDK 9.0.1xx) into vs17.13 (SDK 9.0.2xx)
         "vs17.12": {
+            "MergeToBranch": "vs17.13"
+        }
+        // MSBuild latest release to main
+        "vs17.13": {
             "MergeToBranch": "main"
         }
     }
-}
+}